1. Field of the Invention
This invention relates generally to a primary reactor for a fuel processor system and, more particularly, to a primary reactor for a fuel processor system, where the reactor includes an electrically heated catalyst for improved system start-up.
2. Discussion of the Related Art
Hydrogen is a very attractive source of fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives a hydrogen gas and the cathode receives an oxygen gas. The hydrogen gas is ionized in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode, where they react with the oxygen and the electrons in the cathode to generate water as a by-product. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle. Many fuels cells are typically combined in a fuel cell stack to generate the desired power.
Proton exchange membrane fuel cells (PEMFCs) are a popular fuel cell for vehicles. In a PEMFC, hydrogen (H2) is the anode reactant, i.e., fuel, and oxygen is the cathode reactant, i.e., oxidant. The cathode reactant can be either pure oxygen or air (a mixture of O2 and N2). The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perflurosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an isomer. The combination of the anode, cathode and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacturer and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
In vehicle fuel cell applications, it is desirable to use a liquid fuel, such as alcohols (methanol or ethanol), hydrocarbons (gasoline), and/or mixtures thereof, such as blends of ethanol/methanol and gasoline, as a source of hydrogen for the fuel cell. Usually, hydrocarbon-based liquid fuels are dissociated within a chemical fuel processor system or reformer to release the hydrogen therefrom for fueling the cell. The fuel processor system contains one or more reactors where the fuel is reacted chemically to break down the hydrocarbons in the fuel with water and/or air to generate a reformate gas comprising hydrogen and carbon monoxide, methane, nitrogen, carbon dioxide and water as by-products.
Generally, the reactor is a steam reformer or auto-thermal reactor (ATR). The steam reformer requires an external heat source to generate the heat required to dissociate the hydrocarbon fuel. The ATR includes a partial oxidation (POX) reactor and a steam reformer. The POX reactor includes a catalyst that generates heat by an exothermic reaction to heat the steam reformer and dissociate the hydrocarbon fuel. A steam reformer typically provides a higher conversion percentage of the hydrocarbon fuel into hydrogen than the POX reactor. However, a steam reformer requires a significant heat input than the POX reactor.
The known fuel processor systems also typically include downstream reactors, such as water-gas shift (WGS) reactors and preferential oxidation (PROX) reactors. The WGS and PROX reactors are necessary to convert carbon monoxide (CO) to carbon dioxide (CO2) in the reformate gas because carbon monoxide contaminates the catalytic particles in the PEM fuel cell stack. It is desirable that the carbon monoxide in the reformate gas be less than 100 ppm to be suitable for fuel cell applications. The WGS reactor employs catalysts that convert carbon monoxide and water to carbon dioxide and hydrogen. The PROX reactor employs catalysts that selectively oxidize carbon monoxide (using oxygen from air as an oxidant) in the presence of hydrogen to produce carbon dioxide (CO2).
The reformate gas stream passes through the fuel cell stack that utilizes the hydrogen in the reformate gas and oxygen from air. An anode exhaust gas and a cathode exhaust gas are discharged from the stack. The anode exhaust gas is the anode input gas stream minus the hydrogen used by the stack and the cathode exhaust gas is a depleted oxygen stream. The two exhaust gas streams, in some designs, are then sent to a tail gas combustor, which consumes the anode exhaust gas using oxygen from air or the cathode exhaust gas. The combustor energy can be employed to integrate heat into the fuel processor system, run an expander, run a co-generation process or be exhausted.
FIG. 1 is a plan view of a fuel processor system 10 for generating hydrogen to be used in a fuel cell engine of the type discussed above. A hydrocarbon fuel, such as gasoline, natural gas, methane, propane, methanol and/or mixtures thereof, is fed to a primary reactor 14, such as an ATR, from a suitable source (not shown) on a line 16. The hydrocarbon fuel reacts with a steam/air mixture received on a line 18 from a heat exchanger 20 to dissociate the hydrogen from the fuel and generate a hydrogen-rich reformate gas. The reactor 14 includes a steam reforming and/or partial oxidation catalyst suitable for the specific fuel being used. The operating temperature of the reactor 14 depends on the nature of the fuel and the relative compositions of fuel, air and water, and is typically between 300° C. and 800° C. The reformate gas exiting the primary reactor 14 on a line 44 contains primarily hydrogen, nitrogen, carbon monoxide, carbon dioxide, water and possibly methane.
The steam for the steam/air mixture is generated in a heat exchanger 24, where liquid water provided on a line 26 is heated and vaporized in the heat exchanger 24 by a hot exhaust stream on a line 28 from a combustor 30, such as a tail gas combustor. The steam exits the heat exchanger 24 on a line 34 and is mixed with compressed air provided on a line 36 in a mixing zone or valve 38. The steam/air mixture exits the zone or valve 38 on a line 40 to be sent to the heat exchanger 20 to form the hot steam/air mixture on the line 18 sent to the reactor 14. The heat required to raise the temperature of the steam on the line 40 in the heat exchanger 20 is generated by the reformate gas from the reactor 14 on the line 44. Alternatively, the air and water can be heated separately and mixed either within or before the primary reactor 14.
It is necessary to convert carbon monoxide to carbon dioxide in the reformate gas being used in a fuel cell stack because carbon monoxide contaminates the catalyst particles used therein. The carbon monoxide concentration of the reformate gas on the line 44 is typically between about 5 mole percent and about 20 mole percent. Typically, fuel processing systems employ WGS reactors to reduce the carbon monoxide in the reformate gas flow. The reformate gas on the line 44 is cooled in the heat exchanger 20 to the operational temperature of a WGS reactor 48. The cooled reformate gas is then applied to the WGS reactor 48 on a line 50, where carbon monoxide and water are converted to hydrogen and carbon dioxide by a catalyst reaction process that is well understood in the art. Conventional catalysts, such as Fe3O4/Cr2O3 for high temperature shifts or CuO/ZnO/Al2O3 for low temperature shifts, may be used, as well as any other known WGS catalyst.
The WGS reactor 48 can be a high temperature WGS reactor (320° C.-500° C.), a medium temperature WGS reactor (250° C.-400° C.), or a low temperature WGS reactor (150° C.-250° C.). Alternately, the reactor 48 can include a combination of high, medium and low temperature WGS reactors that employ a technique for cooling the reformate gas as it flows between the different temperature reaction zones. Generally, the temperature of the WGS reactor 48 decreases with the direction of the reformate gas flow.
The WGS reactor 48 generates a reformate gas flow on a line 52 that is primarily hydrogen, nitrogen, carbon monoxide, carbon dioxide and water. The reformate gas will typically include about 0.3-3 mole percent CO depending on the exit temperature of the WGS reactor 48, the space velocity of the reformate gas on the line 50, the steam to carbon ratio and the catalyst used. The reformate gas exits the WGS reactor 48 on the line 52 with less carbon monoxide and more hydrogen than the reformate gas on the line 50. However, the WGS reactor 48 cannot remove enough of the carbon monoxide in the reformate gas for the PEM fuel cell stack. Therefore, the reformate gas on the line 52 is sent to a PROX reactor 54. The operating temperature of the WGS reactor is greater than the operating temperature of the PROX reactor 54. Therefore, the temperature of the reformate gas exiting the WGS reactor 48 is above the operating temperature of the PROX reactor 54. Thus, a heat exchanger 56 is provided to cool the reformate gas on the line 52 to a reduced temperature on a line 58.
The PROX reactor 54 removes more of the carbon monoxide in the reformate gas that would otherwise contaminate the catalytic particles in the PEM fuel cell. The PROX reactor 54 selectively oxidizes carbon monoxide in the presence of hydrogen to produce carbon dioxide (CO2) using oxygen from air as an oxidant. The reformate gas from the PROX reactor 54 is then provided to a fuel cell engine stack 60 on line 62, or is stored as compressed gas in a container for future use. Some primary reactor designs preheat a certain gas, such as nitrogen, that flows through the catalyst monolith to heat the catalyst therein at system start up. However, the known techniques for heating the catalyst monolith in the reactor at system start-up have heretofore been relatively inadequate.
State of the art primary reactors in a fuel processor system typically have a relatively long start-up time before the reactor becomes hot enough to dissociate the hydrocarbon fuel to produce hydrogen. The long start-up time is directly related to the relatively large mass and large volume of the catalyst monoliths in the reactor because of the energy needed to get the catalyst monoliths up to their operating temperature. It is desirable to reduce the start-up time of the fuel processor system by quickly heating the catalysts in the primary reactor when the system is turned on.